Ferritic stainless steel such as JIS (Japanese Industrial Standards)-SUS444 is less sensitive to stress corrosion cracking (SCC) than austenitic stainless steel, and thus has been used as a material of electric water heaters and the like.
However, running water contains residual chlorine which has been added for sanitary requirements, so that ferritic stainless steel used as a material of an electric water heater may be corroded by the oxygen behavior of the residual chlorine. In particular, welds (weld metals) and welded heat affected zones often have problems with corrosion resistance.
To improve corrosion resistance, for example, Japanese Unexamined Patent Application Publication No. 58-71356 discloses a method for improving corrosion resistance through the reduction of P and S, and C and N using a high purity refining technique.
Japanese Unexamined Patent Application Publication No. 10-81940 discloses a technique for improving the corrosion resistance of welds though limitation of Ti content, combined addition of Ti and Al, and addition of a proper amount of Cu.
Japanese Unexamined Patent Application Publication No. 7-286239 discloses ferritic stainless steel with excellent laser weldability, the ferritic stainless steel containing, in terms of % by mass, C≦0.03%, N≦0.025%, O≦0.02%, and 11%≦Cr≦35%, and the contents of C [% C], N [% N], O [% O], and Cr [% Cr] satisfying [% C]+3[% N]+[% O]<(124.4−[% Cr])/1750 such that the oxygen and nitrogen concentrations in the laser welding portions are 250 ppm or less and 350 ppm or less, respectively, the average particle diameter of the precipitated carbide and nitride is 3 μm or less, and the total precipitation density is 1×105/mm2 or less.
Japanese Unexamined Patent Application Publication No. 9-217151 discloses ferritic stainless steel with excellent weldability, the ferritic stainless steel containing; in terms of % by mass, 0.001%≦c≦0.08%, 0.01%≦Si≦1.0%, 0.01%≦Mn≦2.0%, 10.5%≦Cr≦32.0%, 0.001%≦N≦0.04%, 0.005%≦Al≦0.2%, 0.001%≦Mg≦0.02%, and 0.001%≦O≦0.02%, the remainder being composed of Fe and unavoidable impurities.
Japanese Unexamined Patent Application. Publication No. 2005-15816 discloses a can body for a water heater with excellent corrosion resistance, the can body being joined to the upper and lower barreiheads by caulking, the can body being composed of ferritic stainless steel sheet containing, in terms of % by mass, C≦0.003%, 0.1%≦Si≦0.4%, Mn≦0.4%, P≦0.04%, S≦0.01%, 16.0%≦Cr≦25.0%, 0.8%≦Mo≦2.5%, N≦0.03%, 0.1%≦Nb≦0.6%, 0.05%≦Ti≦0.3%, and 0.01%≦Al≦0.5%, the Nb, Ti, C, and N satisfying Nb+Ti≧7(C+N)+0.15, and the remainder substantially being Fe.
Japanese Unexamined Patent Application Publication No. 2006-257544 discloses ferritic stainless steel with excellent crevice corrosion resistance, the ferritic stainless steel containing, in terms of % by mass, 0.001%≦C≦0.02%, 0.001%≦N≦0.02%, 0.01%≦Si≦0.3%, 0.05%≦Mn≦1%, P≦0.04%, 0.15%≦Ni≦3%, 11%≦Cr≦22%, 0.01%≦Ti≦0.5%, and 0.0002%≦Mg≦0.002%, in addition, one or more selected from Mo, Nb, and Cu with percentages of 0.5≦Mo≦3.0%, 0.02%≦Nb≦0.6%, and 0.1%≦Cu≦1.5% within a range satisfying Cr+3Mo+6(Ni+Nb+Cu)≧23, the remainder being Fe and unavoidable impurities.
In recent years, along with tightening of sanitary requirements, Building Health Laws or Building Management Laws were revised in Japan in 2003 to require hot water fed in specific buildings to contain 0.1 mg/L or more of chlorine. As a result of this, in consideration of consumption of the residual chlorine, the chlorine concentration in hot water fed by a hot-water supply system must be increased. Therefore, sufficient corrosive resistance of welds may not be achieved with the known techniques disclosed in Japanese Unexamined Patent Application Publication Nos. 58-71356, 10-81940, 7-286239, 9-217151, 2005-15816, and 2006-257544.
It could therefore be helpful to provide a ferritic stainless steel sheet for a water heater, the steel sheet having sufficient toughness, and providing sufficient corrosion resistance of welds in spite of an increase of chlorine concentration.